Modern automotive vehicle bodies are often equipped with doors that are each fitted with a door beam made of a welded steel pipe or a stamp formed steel plate member to reinforce the vehicle body against side impact. Such an example can be found in Japanese patent laid open publication No. 2002-225561. A door fitted with a door beam is given with an increased mechanical strength and rigidity so that at the time of a side impact not only a significant deformation of the door can be avoided but also the impact energy is favorably transmitted to the vehicle body.
To take full advantage of such reinforced doors, it is desirable to also increase the mechanical strength and rigidity of the vehicle body so that the impact transmitted to the vehicle body may be favorably distributed to the entire vehicle body. As such an effort, it is known for instance from Japanese patent No. 3500626 to install a reinforcement member formed by hydro-forming or the like and provided with a closed cross section to each of the center pillars, roof and floor and weld the reinforcement members with one another so as to define an annular structure as seen from the front end of the vehicle.
According to the vehicle body structure disclosed in Japanese patent No. 3500626, the annular structure increases the mechanical strength and rigidity of the part of the vehicle body surrounding the center pillar, but still leaves a number of problems unresolved. For instance, because the center pillars have a far greater mechanical strength and rigidity than the front pillars and rear pillars, at the time of a side impact, relatively large stress is produced at end portions of each pillar, and such end portions tend to cause sharp bending deformations in the surrounding parts. Therefore, the vehicle body suffers relatively large deformation and the resulting reduction in the overall body rigidity prevents effective absorption of impact energy.
Also, because the reinforcement members for the different parts of the annular structure differ in mechanical strength and rigidity from one another, stress concentrates in the joints between the reinforcement members at the time of a side impact. This again causes sharp bending deformations in such joints, and the resulting reduction in the overall body rigidity prevents effective absorption of impact energy. Such sharp bending deformations can be avoided by locally reinforcing such end portions of each pillar and joints between reinforcing members, but it adds to the weight of the vehicle body and impairs the fuel economy and performance of the vehicle.
A door beam is typically made of high tension steel pipe or high tension steel plate. Such high tension steel material is known to have a high bending strength, but lack the capability to elongate (or to be brittle). This property may prevent a favorable transmission of impact energy from the door to the body depending on the mode of side impact. For instance, when a vehicle having a rigid bumper and frame hits a central part of the door, such a localized impact may fracture the door beam, and it prevents transmission of impact energy from the door beam to the front pillar and center pillar. This problem can be alleviated by increasing the wall thickness and diameter of the door beam, but it again adds to the weight of the vehicle body and impairs the fuel economy and performance of the vehicle.